Environmental Engineering Reference
In-Depth Information
or “brown” tides, so called because the algae are abundant enough to discolor
the water, were described in different marine areas [5], but the Black Sea is
especially rich in such examples. The first description of a large-scale red
tide in the Black Sea, produced by mass development of Dinoflagellate
Pro-
rocentrum cordatum
(syn.
Exuviaella cordata
) was given by Nesterova [10].
In August-September 1973, the density of this species attained 280 million
cells.l
−
1
, representing 95 % of the total phytoplankton biomass. In 1983-1988,
24 cases of “monospecific” blooms in the Romanian NWS zone were recorded.
3. WATER TRANSPARENCY
Increasing in number of unicellular algae and detritus (composed mainly
from dead phytoplankton) is an important reason of changes in marine and
fresh water transparency. The reduction of Black Sea water transparency which
began in the early 1970s [16] is a direct consequence of the increased density of
suspended planktonic organisms, their external metabolites and detritus. The
clouding of seawater, arising from the increased turbidity, especially in the
NWS area, is well known to divers, underwater photographers and fishermen.
Some areas of the Black Sea have now lost their former aesthetic attraction for
tourists and photographers. However, the most serious consequences for the
marine ecosystem were the biological effects, produced by the decreased water
transparency.
Before these events, in the 1960s, Secchi disk readings of 15-18 m were
recorded in offshore NWS waters. At a distance of 1 km from the shore the
readings were 6-7 m. In the 1970s and 1980s, Secchi disk readings were 7-8
m and 2-2.5 m respectively for these two areas [18]. This implies that in the
1960s, the compensation point (the depth below which marine plants release
more organic matter in respiration than they accumulate during photosynthesis
because of the low light intensity) was situated at 45-55 m depth in the offshore
NWS area and at 18-20 m depth in the near-coastal zone. In the 1970s, the
depth of compensation point decreased to 20-25 m in the offshore shelf and to
6-8 m near the coasts.
In the 1970s, light illumination at depths of 25 m and more in the offshore
areas and at depths of 8 m and more in coastal zone became a limiting factor
for bottom macroalgal communities. This was the main reason for the sharp
decline of the famous “Zernov's
Phyllophora
field” (an extensive meadow of
red algae, harvested for the agaroids) in the central part of the NWS. In the
1950s, the area occupied by
Phyllophora
was 10,000 km
2
, with a total biomass
of algae of about 10,000,000 t. Toward 1980s, this area had diminished to 3,000
km
2
and biomass had declined to 1,400,000 t. In the early 1990s, these values
had declined even further to 500 km
2
and 500,000 t respectively [18]. At that
